Voltage Source Converters (VSC), also denoted STATCOM, are today a valuable solution for enhancing power quality of electrical power grids and e.g. for assuring compliance with various requirements in case of integration of stochastic power generation, such as wind power and solar power. A most challenging requirement is the capability of these plants and of the voltage source converters to ride through low or high voltage transients, without tripping voltage breakers and while assuring the grid stability.
Authorities of various countries or regions stipulate various requirements, usually denoted Grid Codes. Riding through of the mentioned low or high voltage transients is typically to not loose active and/or reactive power support during grid faults and especially at fault recovery, when the grid needs the most from compensation equipment such as VSCs.
During a high voltage situation the VSC may experience high electrical stresses (be overloaded) and the requirement of riding through the fault and be controllable after fault recovery may require significant higher equipment costs due to e.g. over-dimensioning of components of the VSC. In particular, the controlled switching of semiconductors of the VSC needs to be blocked upon the DC voltage reaching semiconductor's limit for Switching Safe Operating Area (SSOA), thus reducing the VSC phase to a rectifier type of circuit. DC capacitors of the VSC are then charged by incoming current due to the transient AC overvoltage, thus handling the overvoltage situation.
Today's solutions to the problem are mainly addressed to the conservative design of the VSC, by over-dimensioning the DC capacitors and/or by connecting extra series-connected converter levels in each converter phase. Moreover, DC voltage clamping devices (choppers) may be used at each converter cell of the VSC. All such solutions entail extra costs. Moreover, the VSC is normally blocked for voltage levels typically above 1.4 per unit the nominal bus voltage, eliminating during these blocking periods, the possibility to actively reduce the grid overvoltage. Further, the blocking of the VSC at such high voltage situations entails the risk of individual converter cells exploding, which is costly and possibly dangerous.
In transmission and distribution systems, due to the switching events or resonance conditions, the AC voltage may reach high levels, typically up to 2 per unit the nominal system voltage for short time intervals, typically up to 3 electric periods.